The present invention relates to a motor-driven film winding mechanism, and, more particularly, to a motor-driven film winding mechanism having an improved driving force transmission system for a sprocket drum.
It is well known in the art to provide a motor-driven winding mechanism for a photographic camera either in the form of an attachment or in the form of a built-in mechanism. Such a motor-driven film winding mechanism is adapted to drive a take-up spool, a sprocket drum and a rewinding shaft by a driving motor through driving force transmission systems such as speed reduction gear trains, belt drives and the like.
There has recently been developed a motor-driven film winding mechanism for a camera which can function as an automatic film rewinding apparatus adapted automatically to start the rewinding operation immediately after the completion of exposures and automatically to stop when the film is fully rewound. When rewinding the film with such a motor-driven film winding mechanism, it is necessary to cause the driving motor to rotate in the reverse direction by, for instance, switching the current flow direction to the driving motor or to reverse the motion of the driving force transmission system by, for instance, adding or removing a reversing gear. From the viewpoint of simple construction and low cost, it may be said that the former is quite useful for motor-driven film winding mechanisms in comparison to the latter. Furthermore, the provision of a reversing gear makes the motor-driven film winding mechanism and hence the camera structure complicated and gives rise to great difficulties since a working mechanism for reversing the motion of the driving force transmission system which is mechanically responsive to the completion of exposures has to be additionally provided. Taking such difficulties into consideration, it is advantageous for the motor-driven winding mechanism to cause the driving motor to rotate in the reverse direction in order automatically to rewind the film because this does not require a reversing gear and the working mechanism associated therewith.
Another problem of conventional motor-driven winding mechanisms is that perforations of the film are damaged by excessive forces applied by the sprockets. In general, the sprocket drum of a film winding mechanism, whether it is motor driven or manually operated, is adapted to turn only one complete revolution in order to advance the film by the length of one frame. Now the take-up spool is adapted to attempt to wind up the film a little more than the length by which the sprockets feed the film in consideration of either the leading end of film being loosely grasped on the take-up spool or, in the case of an easy loading arrangement, there being no grasping of the leading end by the take-up spool at the beginning when the film is being moved into the take-up chamber. As winding proceeds, the diameter of the take-up spool increases, yet the film is wound by the same distance each time. In other words, the take-up spool attempts to drive the film a little too fast at the beginning when the diameter is small, and much too fast at the end when it is bulky with film. But as the sprockets drive the film through only a predetermined distance, the take-up spool is usually adapted to slip.
When using an electric motor as a driving source for such a film winding mechanism as is described above, both the take-up spool and the sprockets or sprocket drum have to be forced to rotate by the driving motor in order to wind up the film. Such a film winding mechanism, with the sprocket drum driven by a driving motor, often causes a problem during winding up when the sprockets attempt to drive the film farther even after the completion of exposures. The problem is that, when further winding up the film after the completion of exposures, the advance of the film is arrested because the film trailing end is fixed to the spool of the casette, while the sprockets attempt to rotate since the driving motor is maintained energized in the absence of a signal which is generated each time the film is wound by the length of one frame. If the parts remain in this state, the driving motor has too much load thereon and will burn out, unless before the burning out of the driving motor the film perforations are damaged by the sprockets. As a result of the latter, the sprockets are permitted to rotate further. On the other hand, the condition that the film has been fully exposed cannot be detected.
Moreover, the film at its margin provided with perforations is torn to shreds with which the velvet-protected opening of the cassette may become clogged, and thus a trouble-free rewinding is prevented even if rewinding takes place.
In order to avoid such problems, a conventional motor-driven film rewinding mechanism is provided with a tension-detecting means which causes the driving motor to be disenergized depending on the tension in the film at the time when the film is suddenly restrained. Despite the provision of tension-detecting means, damage to the perforations is unavoidable since the sudden stopping of the film causes the sprockets to crush the edges of the perforations. Therefore it is generally stated that the provision of tension-detecting means cannot fully solve the problem.
Since the motor-driven film winding mechanism described above is so constructed that the sprocket drum and hence the sprockets are driven in the reverse direction at a fixed speed during the rewinding operation, the rewinding shaft has to be slippable according to the increasing diameter of the rewinding spool owing to the fact that a constant amount of the film is driven by the sprockets.
As described above, in motor-driven film winding mechanism, especially having a switching device for causing the driving motor to rotate in the reverse direction for rewinding the film without the provision of a mechanical changeover device, the sprockets tend to damage the film perforations when they attempt to drive the film farther after the completion of exposures, because the sprockets in engagement with the perforations are forced to rotate by the driving motor. In addition, the motor-driven film winding mechanism unavoidably requires the provision of slipping mechanism everywhere desired in order that the same amount of film is driven by either the sprockets and the take-up spool or the sprockets and the rewinding spool. The provision of slipping mechanism is, however, inexpedient from the viewpoint of a complicated structure, as it adds greatly to the expense, has a low driving performance, and has a high electricity consumption.